This invention relates to providing temporary corrosion protection of metallic coated steel using an organic coating. More particularly, the invention relates to metallic coated steel sheet coated with a thin film of siloxane for suppressing white rust.
Metallic coated steel sheet, particularly hot dipped galvanized steel sheet, has long been plagued with the problem of white rusting during storage while awaiting further processing or during shipment to a customer. This white rust is caused by moisture condensation on the surface of the metallic coating.
There have been numerous proposals to suppress white rust caused by moisture condensation on galvanized steels. Several artisans have proposed rinsing galvanized steel in a bath containing up to about 10 wt. % of a silane coupling agent prior to painting. Silanes proposed include aminopropyl trimethoxy, aminopropyl triethoxy, methacryloxy propyl trimethoxy and glycidoxypropyl trimethoxy. The rinsed steel may be baked at an elevated temperature to form a hardened or permanent thick silane coating. Although the thick silane coating provides good corrosion protection, the coating may be difficult to remove prior to subsequent processing.
It also is known to rinse galvanized steels with a chromate solution containing a silane coupling agent to suppress white rust. Although the chromate coating provides good corrosion protection, the coating is formed using environmentally toxic solutions that can not be disposed of economically.
It also is known to use a composite coating to protect galvanized steel from white rust by sequentially rinsing the steel with a chromate solution and a silicate solution. These toxic solutions also can not be disposed of economically and the composite coating generally can not be removed.
It recently has been proposed to protect galvanized steel from white rust using a silica compound. Immediately after hot dipping while still at galvanizing temperatures, a freshly coated steel is rinsed with a solution containing an organic or inorganic alkyl silicate.
As evidenced by the effort of previous workers, there has been a long felt need to develop a low cost, corrosion resistant coating for metallic coated steel that is formed using environmentally safe coating solutions that can be disposed of economically. There especially is a need for a nontoxic, corrosion resistant coating that suppresses white rust and does not impair phosphatability of hot dipped galvanized steels to be painted. Ideally, the corrosion resistant coating should be removable from the steel prior to subsequent processing so that the steel substrate is free of corrosion products, dirt, oil and the like to enhance bonding of paint to the steel substrate.